bird's-eye views. 19 



image formed on the retina is conveyed to the brain, and 

 transformed into a mental perception, capable of being 

 thought about. This is inscrutable ; it is here the part of 

 wisdom to confess ignorance, and acknowledge bounds that 

 human reason cannot overstep. Nor have we need here to 

 go into the general optical laws applicable to vision ; they 

 are well known, and moreover relate no more to a bird's eye 

 than to that of any other animal. What we want is to find 

 out the meaning of the structural peculiarities by which a 

 bird's eye differs from other eyes. What is the reason and 

 the purpose of the three eyelids? of the shape of the ball? 

 of the very abundant aqueous humor? of the movable lens? 

 of the marsupium ? the tapering nerve ? What special rela- 

 tion do these and other features bear to the sense of sight in 

 birds? In other words, why must a bird have just this sort 

 of an eye to be able to see perfectly? Some of these ques- 

 tions can be satisfactorily answered ; others not. Some we 

 have already replied to as they arose in our mind invol- 

 untarily during our dissection. Thus the third lid gives a 

 subdued light, without excluding light altogether ; and also 

 protects the eye, which could not be otherwise protected 

 without closure of its outer opaque lids, and loss of sight 

 altogether. The very convex and highly refractive cornea 

 doubtless has some relation to a bird's ability to see straight 

 ahead, though its eyes look directly sidewise. 



Perhaps no reason has been assigned for the singular 

 course and termination of the optic nerve. These have pos- 

 sibly no special optical relations ; the cause may lie simply 

 in the relative situations of the brain and eye, which are 

 such, that the nerve would have to change its course ab- 

 ruptly to pierce directly through the sclerotic. We cannot 

 see through a crooked tube, any more than we can shoot 

 round a corner. Nerves are the railroads of thousfht. A 

 train of thought might run off the track if the curves and 

 grades were not easy. I believe that we find comparatively 

 few instances of abrupt angles in the course of nerves. If 



